Locally around where I live, 44 mag ammo is more common. If you are not a reloader, I would say go with the 44 magnum.
One thing most people don't seem to consider when choosing between a 44RM or a 45LC, is cartridge rim diameter. I asked around why Winchester never made their 1873 in 45LC, as that would have made a lot of sense back then sharing pistol and rifle ammo just like they did with the 44-40. The consensus seems to be that the 45LC rim diameter is smaller than 44-40, (and 44RM), thus offering less grip area for the extractor, and possibly making a dirty rifle not extract as reliably.
Yeah sure, we all keep our rifles clean, but just incase it gets dirty, why not go with the advantage of a bigger rim, that offers more grip area for your extractor.....
Google research shows 44-40 rim is .054" bigger than the base, 44RM rim is .057" bigger than the base, and 45LC is only .032" bigger than the base.
I am sure there might be a few comments saying this is B.S. but I can only see why this would be an advantage and not a disadvantage.